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Homework Help: Electron transfer in Redox equation

  1. Dec 4, 2008 #1
    1. The problem statement, all variables and given/known data
    How many electrons are transferred in the following reaction?
    6 Br^- (aq) + Cr2O7^2- (aq) + 14 H^+ --> 2Cr^3+ (aq) + 7H2O + 3Br2 (l)

    a) 1
    b) 2
    c) 3
    d) 6
    e) 14

    2. Relevant equations

    3. The attempt at a solution
    Redox reactions really confuse me, but I gave it a shot. I took a look at the number of electrons on each side of the equation and found the following:
    Left side:
    6Br^- = -6
    Cr2O7^2- = -2
    14 H^+ = +14
    Total: +6

    Right side:
    2Cr^3+ = +6
    7H2O = 0
    3 Br2 = 0
    Total: 6

    But this is as far as I can get. Somehow I don't think the answer is D) 6, but I really don't know. Someone please help me understand this problem?
  2. jcsd
  3. Dec 4, 2008 #2


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    Easily concentrate your attention to just one half-reaction, assuming the reaction as written is balanced; that being for the bromide. How many electron difference is it from 2 bromides to one bromine(compound, not the separate atoms)? Now, how many electrons change is this for the same half-reaction as represented in the fully written reaction (in which you start with 6 bromides instead of just 2 bromides)?
  4. Dec 4, 2008 #3


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    In other words, from post #2, your best choice would seem to be D. (6 electrons)
  5. Dec 4, 2008 #4
    Okay, so D it is. If I understand correctly then in the half-reaction for bromide... there is 1 electron difference between 2 bromides to 1 bromine. As a whole reaction, there are 6 bromides... so 6 electrons.

    What about the other half-reaction, though?
    Cr2O7^2- + 14H^+ --> 2Cr^3+ +7H2O

    I suppose it is the dichromate ion that confuses me.
  6. Dec 4, 2008 #5


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    If you know your reaction as written is balanced, then you know that the chromate part of the reaction is also for 6 electrons.

    Look at dicrhromate anion. Account for all the charges which give the -2 charge for this anion. Two chromiums, seven oxygens, the sum of the charges must be -2. You want to find the charge on the chromium. You know: 1 oxygen is -2, dichromate anion charge is -2. You do not yet know the charge on 1 chromium in the dichromate anion. This is what you want to find (using simple algebra).
  7. Dec 4, 2008 #6
    Ahhh... I see. Thank you for your help. I understand now. :) Redox reactions make my head spin.
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